feat(bff): add /health endpoint (refs #28)

Register ASP.NET Core health checks and map GET /health. The endpoint
returns 200 with a "Healthy" body by default, making the red test pass.
Smallest change to go green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/bff/Bff.Api/Program.cs

@@ -1,7 +1,10 @@
 var builder = WebApplication.CreateBuilder(args);
+builder.Services.AddHealthChecks();
+
 var app = builder.Build();
 
 app.MapGet("/", () => "BFF placeholder");
+app.MapHealthChecks("/health");
 
 app.Run();